The present invention relates to an apparatus and method for controlling a gas sensor, which is capable of detecting a specific gas component in a gas under measurement, and more particularly to control of the gas sensor at startup.
Various gas sensors are used to measure the concentration of a specific gas component e.g. nitrogen oxide (NOx) or ammonium in a gas under measurement such as an exhaust gas from an internal combustion engine. For example, there is known a NOx sensor that includes a sensor element equipped with an oxygen concentration detection cell and first and second oxygen pumping cells, each of which has an oxygen ion conducting solid electrolyte layer and a pair of porous electrodes arranged on the solid electrolyte layer, to define first and second measurement chambers. When a gas under measurement first flows into the first measurement chamber, the first oxygen pumping cell pumps oxygen in or out of the first measurement chamber so as to adjust the oxygen concentration of the gas under measurement in the first measurement chamber to a given level and thereby maintain the output voltage of the oxygen concentration detection cell at a constant value. When the gas under measurement flows from the first measurement chamber into the second measurement chamber, the second oxygen pumping cell pumps oxygen out of the second measurement chamber with the application of a constant drive voltage between the porous electrodes so as to produce a flow of electric current through the second oxygen pumping cell according to the amount of oxygen dissociated from NOx in the gas in the second measurement chamber and pumped out by the second oxygen pumping cell. The NOx concentration of the gas under measurement can be thus determined based on the current output of the second oxygen pumping cell.
In the case of using this type of NOx sensor to measure the NOx concentration in the engine exhaust gas, the second measurement chamber changes into a lean state (close to the air atmosphere) during a lapse of time from the end of the previous operation to the restart of the engine. In order to bring the second measurement chamber into a low oxygen concentration state quickly at startup of the sensor element and shorten the stabilization time required for the NOx sensor to become ready for stable NOx concentration measurement, Japanese Laid-Open Patent Publication No. 2001-281211 and No. 2001-141696 propose so-called preliminary drive control of the second oxygen pumping cell to pump oxygen out of the second measurement chamber forcefully with the application of a constant voltage higher than that in normal drive control.